Increasing oil prices have heightened the importance of developing cost effective renewable energy. Significant efforts are underway around the world to develop cost effective solar cells to harvest solar energy. In order for solar cells to be cost effective with traditional sources of energy solar cells must be manufactured at a cost well below $1/watt.
Current solar energy technologies can be broadly categorized as crystalline silicon and thin film technologies. Approximately 90% of the solar cells are made from silicon—single crystal silicon or polycrystalline silicon. Crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar modules with good efficiencies (13-18%, half to two-thirds of the theoretical maximum) and uses process technology developed from the knowledge base of the microelectronics industry. Silicon solar cells are very expensive with manufacturing cost above $3.50/watt.
Second generation solar cell technology is based on thin films. Main thin film technologies are Amorphous Silicon, Copper Indium Gallium Selenide (CIGS), and Cadmium Telluride (CdTe).
Amorphous silicon (a-Si) was viewed as the “only” thin film PV material in the 1980s. But by the end of that decade, and in the early 1990s, it was written off by many observers for its low efficiencies and instability. However, amorphous silicon technology has made good progress toward developing a very sophisticated solution to these problems: multijunction configurations. Now, commercial, multijunction a-Si modules in the 7-9% efficiency range are being produced by several companies. A number of companies such as Kaneka, Sharp, Schott Solar, Ersol, etc., are manufacturing amorphous silicon solar cells on glass substrates by adopting commercially proven CVD process to deposit a-Si originally developed for flat panel display manufacturing. Equipment companies such as Applied Materials are offering turn-key systems to manufacture a-Si solar cells on glass substrates. The key obstacles to a-Si technology are low efficiencies, light-induced efficiency degradation (which requires more complicated cell designs such as multiple junctions), and process costs (fabrication methods are vacuum-based and fairly slow). United Solar has pioneered triple junction a-Si solar cells on flexible stainless steel substrates. However, a-Si solar cells are expensive to manufacture (>$2.5/watt).
Thin film solar cells made from Copper Indium Gallium Diselenide (CIGS) absorbers show promise in achieving high conversion efficiencies of 10-12%. The record high efficiency of CIGS solar cells (19.9% NREL) is by far the highest compared with those achieved by other thin film technologies. These record breaking small area devices have been fabricated using vacuum evaporation techniques which are capital intensive and quite costly. A number of companies (Honda, Showa Shell, Wurth Solar, Nanosolar, Miasole etc.) are developing CIGS solar cells on glass substrates and flexible substrates. However, it is very challenging to fabricate CIGS thin films of uniform composition on large area substrates. This limitation also affects the process yield, which are generally quite low. Because of these limitations, implementation of evaporation techniques has not been successful for large-scale, low-cost commercial production of CIGS solar cells. It is extremely unlikely that CIGS solar cells can be produced below $1/watt manufacturing cost.
CdTe thin film solar cells are very simple to make and have the potential to achieve lowest manufacturing cost compared to all other solar cell technologies. CdTe solar cells with 16.5% efficiency have been demonstrated by NREL. First Solar based in Arizona is producing CdTe solar cells on glass substrates at a manufacturing cost of $1.12/watt. First Solar expects to reduce the cost to below $1/watt by the end of 2009 when it ramps up its annual manufacturing capacity to 1 GW. Further reduction in manufacturing cost of CdTe solar cells is not readily achievable because of relatively slow piece by piece manufacturing process.
CdTe solar cells are made by depositing CdTe on 3 mm thick glass substrates and encapsulated with a second 3 mm cover glass. Hence they are produced by a slow piece by piece manufacturing process. Further reduction in manufacturing cost of CdTe solar cells to well below $1/watt is not readily achievable because of slow piece by piece manufacturing process. These CdTe solar cells are also very heavy and cannot be used for residential rooftop applications—one of the largest market segments of solar industry. Opportunity exists to innovate by developing CdTe solar cell on flexible substrate that can be manufactured by a continuous roll to roll process to significantly reduce manufacturing cost. Flexible solar cells will also be light weight making them suitable for residential roof top applications which are not accessible to CdTe on heavy glass substrates.
Superstrate solar cell configurations are known in the art and have a configuration comprising a substrate which is transparent and faces the sun to generate photovoltaic output. A transparent contact, commonly Indium doped tin oxide (ITO) or Fluorine doped tin oxide (FTO) film is deposited on the substrate followed by CdS window layer. In general the transparent conducting oxide (TCO) is a bilayer consisting of a 200-400 nm thick highly conducting layer with sheet resistance 5-10 Ω/cm3 and a 20-50 nm thick resistive layer with resistance 1-2 Ω-cm. A junction is formed by depositing a CdTe absorber layer using deposition techniques such as close spaced sublimation (CSS), sputtering, electrodeposition, screen printing & sintering or spray pyrolysis. The substrate temperature varies from one deposition technique to other; >600° C. for CSS and ˜100° C. for electrodeposition. CdCl2 treatments, generally at 400° C., are carried out on this device structure to improve the grain size and electronic properties. During the high temperature junction formation or the subsequent CdCl2 treatment significant CdTe—CdS inter-diffusion is observed, which enable the fabrication of high efficiency devices. CdCl2 treatment is followed by the contact treatments to form pseudo-ohmic contact to CdTe. Due to its high work function and it is not possible to dope CdTe>1016 cm3 and also there are no metals available that have a work function higher than CdTe, it is not possible to form ohmic contact to CdTe without the contact treatments. Contact treatments involves either etch treatments such as Br-Methanol or Nitric-Phosphoric acid (NP) or the deposition of interface layers such as Cu2Te, Sb2Te3, Bi2Te3, CuZnTe, HgCdTe that can be doped P+ to form pseudo ohmic or tunneling contacts. Metal electrodes are deposited on surface treated CdTe films using known techniques.
Substrate configuration solar cells are required when opaque substrates such as metal foil substrates are used for high volume production of CdTe/CdS devices. This change in the device configuration necessitates a substantial deviation from the conventional junction formation processing. Prior art substrate CdS/CdTe device performance is inferior to superstrate prior art devices. This is a result of the process advantages associated with the superstrate configuration such as enhanced CdS—CdTe inter-diffusion at higher CdTe processing temperature and the availability of post deposition CdTe surface for ohmic contact processing.